Tyrosine kinase inhibitors (TKIs) against oncogenic tyrosine kinases (OTK; e.g. BCR-ABL1, EGFRL858R, ERBB2) have created a new era of treatment for OTK leukemia and solid tumors and have substantially improved clinical outcome. Despite their clinical success in chronic myeloid leukemia (CML), resistance to TKIs, such as Imatinib, is a common outcome in most OTK-malignancies. While combination of TKI with conventional chemotherapy results in prolonged survival for most OTK-malignancies, for a large group of patients, no curative treatment option is available. In preliminary work for this proposal we have found that two negative feedback regulators (SPRY2 and DUSP6) are expressed at high levels in OTK-malignancies but not in normal cells or tumor cells lacking an oncogenic tyrosine kinase. The inhibitory linker molecule Sprouty2 (SPRY2) attenuates signal transduction immediately downstream of the oncogenic tyrosine kinase, the Dual Specific Phosphatase DUSP6 interferes with distal signal transduction by dephosphorylation of ERK1/2. Expression levels of these molecules are dependent on the signaling strength of the oncogenic tyrosine kinase and are completely suppressed upon TKI-treatment. Molecules of the Sprouty- and DUSP- families of negative feedback mediators are often inactivated through deletion or promoter hypermethylation in Non- tyrosine kinase tumors but not in OTK-malignancies. Given that SPRY2- and DUSP6-mediated negative feedback signaling was highly active in all OTK-tumor samples studied, we performed experiments that demonstrated that OTK-leukemia cells indeed critically depend on continuous active negative feedback signaling to calibrate otherwise overwhelming tyrosine kinase signaling. In this proposal, we will test the central hypothesis that negative feedback regulators represent a fundamentally novel class of therapeutic targets for the treatment of OTK-leukemias and -solid tumors. The central goal of this proposal is to build a fundamental understanding of how blockade of negative feedback mechanistically leads to cell death in OTK-malignancies (Aims 1-2), and to leverage this mechanistic information towards the development of a new therapy concept based on alternating treatment schedules between TKIs and negative feedback inhibitors (NFIs; Aim 3).